Stable compositions for inhibiting polymerization of unsaturated carboxylic acid esters

ABSTRACT

A method of inhibiting polymerization of unsaturated carboxylic acid esters and improved unsaturated carboxylic acid ester compositions are described. The method comprises, and the compositions are prepared by, incorporating into the ester composition a N-(aminoalkyl)phenylenediamine of the formula   IN WHICH R, R1 and R2 independently are hydrogen or lower alkyl, X is hydrogen, chloro, trichloromethyl, trifluoromethyl, lower alkyl, lower alkoxy or phenoxy and n is 1-5.

United States Patent [1 1 Sullivan et a1.

[ Dec. 31, 1974 STABLE COMPOSITIONS FOR INI-IIBITING POLYMERIZATION OF UNSATURATED CARBOXYLIC ACID ESTERS [22] Filed: Jan. 23, 1974 [21] Appl. No.: 435,718

[52] Cl 260/486 R, 260/398.5, 260/485 S [51] Int. Cl.. C07c 69/54 [58] Field of Search 260/486 R, 485 S, 398.5

[56] References Cited UNITED STATES PATENTS 2,344,798 3/1944 Brown et a1 260/486 R 2,815,369 12/1957 Holt 260/486 Primary Examiner-Lorraine A. Weinberger v Assistant Examiner-Paul J. Killos [5 7] ABSTRACT A method of inhibiting polymerization of unsaturated carboxylic acid esters and improved unsaturated carboxylic acid ester compositions are described. The method comprises, and the compositions are prepared by, incorporating into the ester composition a N,-

1 (aminoalkyl)phenylenediamine of the formula I @NHQNHCHZ -c- NH-R in which R, R and R independently are hydrogen or lower alkyl, X is hydrogen, chloro, trichloromethyl, trifluoromethyl, lower alkyl, lower alkoxy or phenoxy and n is 1-5. 1 v

6 Claims, No Drawings STABLE COMPOSITIONS FOR INII IBITING POLYMERIZATION or UNSATURATED CARBOXYLIC ACID EYSTERS BACKGROUND OF THE INVENTION This invention relates to methods of inhibiting polymerization of unsaturated monomers, more particularly it relates to methods'of inhibiting polymerization of unsaturated carboxylic acid esters and to improved unsaturated carboxylic acid ester compositions. Processes concerning the preparation and the stabilization of unsaturated carboxylic esters are found in Patent Office Class 260, subclass .486.

. Substantial quantities of unsaturated carboxylic acid esters, for example methyl methacrylate, are manufachired for use as intermediates in the production of polymers and copolymers. During manufacture, transportation and storage of these esters, it is essential that polymerization of these monomers is avoided. If premature polymerization occurs during manufacture,the polymer fouls or completely plugs production equipment and transfer lines which fouling leadsto expensive dismantling and cleaning of production facilities.

The problem is particularly acute during the distillation step which step is required to isolate the desired ester monomer. lf'premature polymerization occurs during transportation or storage, recovery of the polymer from the storage vessel is extremely difficult. Of course, any

. SUMMARY OF THE INVENTION A class of N-(aminoalltyl)-N'-phenyl-pphenyle'nediamines has been found which possesses enhanced activity in respect to inhibiting the polymeriza- The terms used herein and in the claimsmean as follows: Lower alkyl" means an alkyl radical derived tion of unsaturated carboxylic acid esters. The inhibitors ofthis invention are characterizedby the formula I l o o X in which R,'R,, and R independently are hydrogen or lower alkyl, X is hydrogemchl'oro, t richloromethyl, tri

fluoromethyl, lower alkyl, lower alkoxy or. phenoxy and n is 1 5. Inhibitors in which n is one, X and R are hydrogen and R and R are lower alkyl comprise a pre-' ferred subclass. The presence of an amino group on the alkyl radical attached to nitrogen of the phenylenediamine enhances the inhibitor activity. The presence of substituents attached to the phenyl radical of the phenylenediamine designated by X above has little effect upon inhibitor activity.

from removal ofone hydrogen atom from alkane of 1-5 carbon atoms. Straight chain unbranched radicals are preferred. Illustrative examples of satisfactory lower alkyl radicals are methyl, ethyl, pr'opyl, isopropyl, nbutyl, isobutyl, sec-butyl, tert-butyl, pentyl and isopentyl with methyl being the preferred radical. Alkoxy comprises an alkyl radical attached to the remainder of the molecule by oxygen.

The term unsaturated carboxylic acid esters is used in the generic sense and means aliphatic esters of unsaturated mon,o-, di-, and tricarboxylic acids. Lower alkyl esters of ethylenically unsaturated'monocarboxylic acids are preferred. The size of the carboxylic acid or ester moiety is immaterialsPolymerization of esters of unsaturated carboxylic acids of30 carbon atoms or more are inhibitedwith the inhibitors of this invention. Illustrative examples of compounds stabilized with the 7 inhibitors of this invention are the methyl, ethyl, butyl,

and 2-ethylhexyl esters of the following acids: acrylic,

methacrylic, angelic, crotonic, isocrotonic, propynoic,

sorbic, oleic. elaidic, linoleic, a-eleostearic, .B-eleostearic, a-linolenic anderucic. Other examples are the dimethyl', diethyl, dibutyL'and di(2-ethylhexyl) esters of maleic acid, fumaric acid, itaconic acid and acetyle'nedicarboxylic acid, and trialkyl esters of aconitic-(1,2,3-propenetricarboxylic) acid. A preferred subgroup of unsaturated carboxylic acid esters are acrylate. esters derived from acrylic and methacrylic acids. These esters are characterized by the formula CH =C(R )C(O)OR in which R, is hydrogen or methyl and R is alkyl of l-8 carbon atoms. Lower alkyl esters are preferred.

The inhibitors of this invention are prepared by known procedures. For example, N-(am'inoalkyl)-N- (phenyl)-p-phenylenediamines are prepared by reduc-- tion of N-(nitroalkyl)-N-phenyl-p-phenylenediamines which N-(nitroalkyl)-N'-phenyl-p-phenylenediamines are. prepared by reacting a nitroalkane and 4- aminodiphenylamine with formaldehyde or by reacting a 2'-nitro alcohol 'with'4 aminodiphenylamine.

Illustrative inhibitors of the invention are: N-(2-- methyl-Z-aminobutyl)-N'-phenyl-p-phenylenediamine N-(2-methyl-2-aminopentyl)-N-phenyl-pphenylenediamine N-(Z-ethyl-Z-aminobutyl)-N-phenyl-pphenylenediamine N-(2,3,3-trimethyl-2-aminobutyl)-N-phenyl-pphenylenediamine N-(2ethyl-2-aminopentyl)-N'-phenyl-pphenylenediamine .N-(2-propyl-2-aminopentyl)-N'-phenyl-pphenylenediamine N-(2-propyl-2-aminohexyl)-N-phenyl-pphenylenediamine N-(2-isopropyl-3,3-dimethyl-2-amin0butyl)-N' phenyl-p-phenylenediamine N-(2-methyl-2-methylaminobutyl)-N'-phenyl-pphenylenediamine N-(Z-methyl-Z-methylaminopentyl)-N'-phenyl-pphenylenediamine N-(2-ethyl-2-methylaminobutyl)-N-phenyl-pphenylenediamine N-(2,3,3-trimethyl-2-methylaminobutyl )-N -ph 1 p-phenylenediamine 3 N-(Z-ethyl-2-methylaminopentyl)-N'-phenyl-pphenylenediamine N-(2-propyl-2-methylaminopentyl)-N'-phenyl-pphenylenediamine N-(2-propyI-2-methylaminohexyl)-N-phenyl-pphenylenediamine N-(2-isopropyl-3,3- dimethyl-Z-methylaminobutyl)- N-phenyl-p-phenylenediamine N-(2-methyl-2-ethylaminobutyl)-N-phenyl-pphenylenediamine N-(2-methyl-2-ethylaminobutyl)-N'-phenyl-pphenylenediamine N-(2-ethyl-2-ethylaminobutyl)-N'-phenyl-pphenylenediamine N-(2,3,3-trimethyl-2-ethylaminobutyl)-N'-phenyl-pphenylenediamine N-(2-ethyl-2-ethylaminopentyl)-N'-phenyl-pphenylenediamine N-(2-propyl-2-ethylaminopentyl)-N-phenyl-pphenylenediamine N-(2-propyl-2-ethylaminohexyl)-N-phenyl-pphenylenediamine N-(2-isopropyl-3,3-dimethyl-2-ethylaminobutyl)-N- phenyl-p-phenylenediamine N-(2-methyl-2-n-propylaminobutyl)-N'-phenyl-pphenylenediamine N-(2-methyl-2-n-propylaminopentyl)-N-phenyl-pphenylenediamine I A N-(2-ethyl-2-n-propylaminobutyl)-N-phenyl-pphenylenediamine N-(2,3,3-trimethyl-2-n-propylaminobutyl)-N'-phenyl-p-phenylenediamine N-(2-ethyl-2-n-propylaminopentyl)-N-phenyl-pphenylenediamine N-(2#propyl-2-n-propylaminopentyl)-N-phenyl-pphenylenediamine N-(2-propyl-2-n-propylaminohexyl)-N'-phenyl-pphenylenediamine N-(2-isopropyl-3,3-dimethyl-2-n-propylaminobutyl)- N-phenyl-p-phenylenediamine v N-(2-aminoethyl)-N-phenyl-p-phenylenediamine N-(3-amin'opropyl)-N'-phenyl-p-phenylenediamine N-(Z-aminopropyl)-N'-phenyl-p-phenylenediamine N-(3-amino-2-me'thylpropyl)-N-phenyl-pphenylenediamine The inhibitors are incorporated into unsaturated carboxylic acid ester compositions by simple addition. Normally, the inhibitor is added to the reactor after the ester is prepared but prior to'distilling the reaction mixture to isolate the unsaturated ester from by-products and reaction media. Generally, additional quantities of inhibitor are added to the ester fraction recovered after distillation to inhibit polymerization during storage. Although the effect of the inhibitors are discussed in terms of inhibiting polymerization, it is probably more accurate to think of the inhibition effect as extending the induction period prior to the onset of polymerization rather than affecting polymerization rate. Regardless of the mode of operation, the consequence of the inhibitor addition is a more stable unsaturated carboxylic acid ester monomer composition which remains in monomer form longer than a similar ester composition without inhibitor added.

The inhibitor effect is concentration dependent, i.e.. the more inhibitor added the longer the time period before the onset of polymerization. The N-phenyl-pphenylenediamine inhibitors of this invention are particularly potentfQuantities of one part per million by weight or less based upon the weight of ester are sufficient to inhibit polymerization. Normally. 5-50 parts per million arerecommended for most applications with amounts 50-200 parts per million being used sometimes and 200-5,000 parts per million being used where more severe conditions are encountered which require greater inhibition. Although even larger quantities are effective, higher dosages are usually unnecessary and are avoided for reason of economy. The amount required for any desired induction time may be readily determined by the methods hereinafter described.

The onset time," the time from the beginning ofthe test until polymerization begins, may be conveniently determined either by measuring the temperature of a sample to detect the liberation of heat which takes place when polymerization begins or by measuring the change in volume of a sample to detect the volume reduction which takes place as polymerization proceeds. Either method is applicable to uncatalyzed samples or samples to which polymerization initiator is added. The exothermic method is described by Bockstahler, et 211., Ind. and Eng. Chem., 50 (10), 1581. The dilatometric method is described by Caldwell and lhrig, J. Am. Chem. 800., 84, 2886.

The inhibitors of this invention are evaluated by the exothermic method essentially as described by Bockstahler, et al., supra. The time required fora test monomer to begin to polymerize is determined at elevated temperature. The test is based on the principle that polymerization is exothermic so that initiation is detected by observing temperature change between a test sample and a stable reference sample maintained in the same environment. The procedure comprises placing a test tube (adapted to accommodate a thermocouple) containing a measured amount of monomer in a constanttemperature bath (maintained within i0.25C). A similar test tube containing silicone oil is used as a reference sample. A differential thermocouple continuously measures the difference inv the temperature AT between the test and reference samples. All samples are measured inthe dark to eliminate any effects due to light. The thermocoupleoutput is recorded on a strip recorder thereby providing a record of AT versus time. When polymerization occurs a sharp deflection in AT is observed from-which the onset time is determined.

DESCRlPTlON OF PREFERRED EMBODIMENTS Satisfactory procedures for preparing the inhibitors of the invention are illustrated below: I

A solution of 2-methyl-2-nitro-l-propanol, 119.] grams (1.0 mole), in 300 ml of ethanol is added dropwise over a period of three hours to a stirred refluxing solution of 4-aminodiphenylamine, 184 grams (1.0 mole), and potassium hydroxide 4 grams, in 200 ml of ethanol. The mixture is stirred at reflux for 24 additional hours; After cooling to 0C, the precipitate is recovered by filtration and air-dried. N-(2-methyl-2- nitropropyl)-N-phenyl-p-phenylenediamine is recovered, 237 grams (83% yield), which after recrystallization from alcohol melts atl27-l28C.

To a Parr bomb, there are charged 69 grams of N-(2 methyl-Z-nitropropyl)-N'-phenyl-p-phenylcnediamine, 12 grams of catalyst comprising W0 platinum on carbon, and [50 grams of xylene. The mixture is heated at 70Cll0C with 200-300 psi hydrogen for one hour.

After cooling to 60C, the catalyst is separated by filtration. The solvent is stripped from the filtrate by vacuum distillation at 190C each4-5 mm Hg. The crude product is distilled to give N-(2-methyl-2-aminopropyl)-N- phenyl-p-phenylenediamine, bLp. 273279C each 6 mm Hg, a white solid, mp. 60-62C.

Ethylmethacrylate is the test monomer used to illustrate the effect of the inhibitors of the invention. A supply of ethylmethacrylate containing hydroquinone inhibitor is repeatedly washed with 0.5 N-sodium hydroxide solution and finally with water to remove all hydroquinone inhibitor from the material. Fifty grams of the unstabilized ethylmethacrylate is added to an above described test tube. A solution ofinhibitor is prepared by adding a carefully weighed quantity of a N- (aminoalkyl)-N-phenyl-p-phenylenediamine to 10 ml of unstabilized ethylmethacrylate. The appropriate volume of inhibitor solution is then added to the test sample with a micropipette to give an inhibitor concentration of 5 parts per million. The test is placed in a constant temperature bath at 85C and the onset time is determined as previously described. The onset time is recorded in hours. -When the test is repeated, the average onset time isreported. The results are shown in Table I. Sample 1 is a control consisting of ethylmethacrylate with no inhibitor present Sample 2 contains a N- (alltyl) N phenyl)-p-phe nyleriemamine inhibitor. Sample 3 illustrates an inhibitor of the invention.

p-phenylenediaminc The data demonstrate that the presence 'of the amino-alkyl group enhances the inhibiting activity of the N-(phenyl)-p-phenylenediamine,.

Although the invention has been illustrated by typical examples, it is not limited thereto. Changes and modifications of the examples of the invention herein chosen for purposes of disclosure can be made which donot constitute departure from the spirit and scope of the invention.

The embodiments of theinvention in which an exclusive property or privilege is claimed are defined as follows:

l. A monomer composition consisting essentially of unsaturated carboxylic acid ester and, in amount effective to inhibit polymerization of the ester, an inhibitor of the formula in which R, R and R independently are hydrogen or lower alkyl, X is hydrogen, chloro, trichloromethyl, trifluoromethyl, lower alkyl, lower alkoxy or phenoxy and n is 1-5.

2. The composition of claim 1 in which the ester is a lower alkyl ester of an ethylenically unsaturated monocarboxylic acid.

3. The composition of claim 2 in which the amount of inhibitor is at least 5 parts per million parts of ester.

acrylate. 

1. A MONOMER COMPOSITION CONSISTING ESSENTIALLY OF UNSATURATED CARBOXYLIC ACID ESTER AND, IN AMOUNT EFFECTIVE TO INHIBIT POLYMERIZATION OF THE ESTER, AN INHIBITOR OF THE FORMULA
 2. The composition of claim 1 in which the ester is a lower alkyl ester of an ethylenically unsaturated monocarboxylic acid.
 3. The composition of claim 2 in which the amount of inhibitor is at least 5 parts per million parts of ester.
 4. The composition of claim 3 in which X and R are hydrogen, R1 and R2 are lower alkyl and n is one.
 5. The composition of claim 4 in which R1 and R2 are methyl.
 6. The composition of claim 3 in which the ester is an acrylate. 